Single-stage power supply with power factor correction and constant current output

ABSTRACT

An example power supply includes an energy transfer element, a switch, and a controller. The switch is coupled to the energy transfer element to control a transfer of energy from an input to a galvanically isolated output of the power supply. The controller includes a delayed ramp generator, an integrator, an arithmetic operator, and a drive signal generator. The integrator generates a first signal responsive to integrating an input current of the power supply. The arithmetic operator generates a second signal responsive to the first signal and responsive to a ratio of a rectified input voltage to a dc output voltage of the power supply. The drive signal generator generates a drive signal in response to a delayed ramp signal and the second signal to control switching of the switch to provide power factor correction of the power supply and to provide a regulated current at the output.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.13/629,334, filed Sep. 27, 2012, now pending, which is a continuation ofU.S. patent application Ser. No. 13/329,009, filed Dec. 16, 2011, nowU.S. Pat. No. 8,305,787, which is a continuation of U.S. patentapplication Ser. No. 12/477,010, filed Jun. 2, 2009, now U.S. Pat. No.8,098,506. U.S. patent application Ser. No. 13/629,334 and U.S. Pat.Nos. 8,098,506 and 8,305,787 are hereby incorporated by reference.

BACKGROUND INFORMATION

1. Field of the Disclosure

The present invention relates generally to power supplies, and morespecifically, the invention relates to control circuits to regulate anoutput of a power supply.

2. Background

In a typical switched-mode power supply application, the ac-dc powersupply receives an input that is between 100 and 240 volts rms (rootmean square) from an ordinary ac electrical outlet. Switches in thepower supply are switched on and off by a control circuit to provide aregulated output that may be suitable for providing current to, forexample, light emitting diodes (LEDs) for illumination. The regulatedoutput is typically a regulated dc current, and the voltage at the LEDsis typically less than 40 volts.

An ac-dc power supply that provides regulated current to LEDs typicallymust meet requirements for power factor, galvanic isolation, andefficiency, as explained below. Designers are challenged to providesatisfactory solutions at the lowest cost.

The electrical outlet provides an ac voltage that has a waveformconforming to standards of magnitude, frequency, and harmonic content.The current drawn from the outlet, however, is determined by thecharacteristics of the power supply that receives the ac voltage. Inmany applications, regulatory agencies set standards for particularcharacteristics of the current that may be drawn from the ac electricaloutlet. For example, a standard may set limits on the magnitudes ofspecific frequency components of the ac current. In another example, astandard may limit the rms value of the current in accordance with theamount of power that the outlet provides. Power in this context is therate at which energy is consumed, typically measured in the units ofwatts.

Power factor is a measure of how closely the ac current approaches theideal. The power factor is simply the power from the outlet divided bythe product of the rms current multiplied by the rms voltage. A powerfactor of 100% is ideal. Currents that have frequency components otherthan the fundamental frequency of the ac voltage will yield a powerfactor less than 100% because such components increase the rms value butthey do not contribute to the output power. The fundamental frequency ofthe ac voltage is typically either 50 Hz or 60 Hz in different regionsof the world. By way of example, the fundamental frequency of the acvoltage is nominally 60 Hz in North America and Taiwan, but it is 50 Hzin Europe and China.

Since the power supply that receives the ac voltage determines thecharacteristics of the ac current, power supplies often use specialactive circuits at their inputs to maintain a high power factor. Powersupplies that use only ordinary passive rectifier circuits at theirinputs typically have low power factors that in some examples are lessthan 50%, whereas a power factor substantially greater than 90% istypically required to meet the standards for input current, such as forexample the International Electrotechnical Commission (IEC) standard IED61000-3-2. Although regulatory agencies in some regions may impose thestandards, manufacturers of consumer equipment often voluntarily designtheir products to meet or to exceed standards for power factor toachieve a competitive advantage. Therefore, ac-dc power supplies forLEDs, for example, typically must include power factor correction.

Safety agencies generally require the power supply to provide galvanicisolation between input and output. Galvanic isolation prevents dccurrent from flowing between input and output of the power supply. Inother words, a high dc voltage applied between an input terminal and anoutput terminal of a power supply with galvanic isolation will produceno dc current between the input terminal and the output terminal of thepower supply. The requirement for galvanic isolation is a complicationthat contributes to the cost of the power supply.

A power supply with galvanic isolation must maintain an isolationbarrier that electrically separates the input from the output. Energymust be transferred across the isolation barrier to provide power to theoutput, and information in the form of feedback signals in many cases istransferred across the isolation barrier to regulate the output.Galvanic isolation is typically achieved with electromagnetic andelectro-optical devices. Electromagnetic devices such as transformersand coupled inductors are generally used to transfer energy betweeninput and output to provide output power, whereas electro-opticaldevices are generally used to transfer signals between output and inputto control the transfer of energy between input and output.

A common solution to provide high power factor for an ac-dc power supplywith galvanic isolation uses two stages of power conversion: One stagewithout galvanic isolation shapes the ac input current to maintain ahigh power factor, providing an intermediate output to a second stage ofpower conversion that has galvanic isolation with control circuitry toregulate a final output. The use of more than one stage of powerconversion increases the cost and complexity of the system.

Efforts to reduce the cost of the power supply have focused on theelimination of electro-optical devices and their associated circuits.Alternative solutions generally use a single energy transfer elementwith multiple windings such as, for example, a transformer or, forexample, a coupled inductor to provide energy to the output and also toobtain the information necessary to control the output. The lowest costconfiguration typically places the control circuit and a high voltageswitch on the input side of the isolation barrier. The controllerobtains information about the output indirectly from observation of avoltage at a winding of the energy transfer element. The winding thatprovides the information is also on the input side of the isolationbarrier. To reduce cost and complexity further, the controller can alsouse the same winding of the energy transfer element to provide energy tothe controller and also obtain information about the input to the powersupply.

The input side of the isolation barrier is sometimes referred to as theprimary side, and the output side of the isolation barrier is sometimesreferred to as the secondary side. Windings of the energy transferelement that are not galvanically isolated from the primary side arealso primary side windings, sometimes called primary referencedwindings. A winding on the primary side that is coupled to an inputvoltage and receives energy from the input voltage is sometimes referredto simply as the primary winding. Other primary referenced windings thatdeliver energy to circuits on the primary side may have names thatdescribe their principal function, such as for example a bias winding,or for example a sense winding. Windings that are galvanically isolatedfrom the primary side windings are secondary side windings, sometimescalled output windings.

While it is quite straightforward to use a winding on the input side ofthe isolation barrier to obtain information indirectly about agalvanically isolated output voltage, it is a different challenge toobtain information indirectly about a galvanically isolated outputcurrent. In many power supply topologies, the measurement of a currentin an input winding alone is not sufficient to determine an outputcurrent. Conventional solutions for measuring an output current usuallyinclude a current to voltage conversion that wastes power and usescostly components to transmit a signal across the isolation barrier.Therefore, conventional solutions are not satisfactory to meet the goalsof galvanic isolation with high efficiency and high power factor at lowcost in an ac-dc converter.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments and examples of the presentinvention are described with reference to the following figures, whereinlike reference numerals refer to like parts throughout the various viewsunless otherwise specified.

FIG. 1 is a functional block diagram of an ac-dc power supply includinga controller for maintaining a high power factor while regulating anoutput current, in accordance with the teachings of the presentinvention.

FIG. 2 is a functional block diagram of an ac-dc power supply includingan alternative controller for maintaining a high power factor whileregulating an output current, in accordance with the teachings of thepresent invention.

FIG. 3 is a schematic diagram illustrating an example arithmeticoperator circuit, in accordance with the teaching of the presentinvention.

FIG. 4 is a timing diagram that shows waveforms of signals from thecircuits of FIG. 1 and FIG. 2.

FIG. 5 is a functional block diagram illustrating an example ac-dcflyback power supply including an alternative controller that provides ahigh power factor while regulating an output current, in accordance withthe teaching of the present invention.

FIG. 6 is a flow diagram illustrating a method to control a single-stageac-dc power supply that provides a high power factor while regulating anoutput current, in accordance with the present invention.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth inorder to provide a thorough understanding of the present invention. Itwill be apparent, however, to one having ordinary skill in the art thatthe specific detail need not be employed to practice the presentinvention. In other instances, well-known materials or methods have notbeen described in detail in order to avoid obscuring the presentinvention.

Reference throughout this specification to “one embodiment”, “anembodiment”, “one example” or “an example” means that a particularfeature, structure or characteristic described in connection with theembodiment or example is included in at least one embodiment of thepresent invention. Thus, appearances of the phrases “in one embodiment”,“in an embodiment”, “one example” or “an example” in various placesthroughout this specification are not necessarily all referring to thesame embodiment or example. Furthermore, the particular features,structures or characteristics may be combined in any suitablecombinations and/or subcombinations in one or more embodiments orexamples. In addition, it is appreciated that the figures providedherewith are for explanation purposes to persons ordinarily skilled inthe art and that the drawings are not necessarily drawn to scale.

The functional block diagram of FIG. 1 shows one example of an ac-dcpower supply 100 receiving an ac input voltage V_(AC) 102 that has asubstantially sinusoidal waveform with a period T_(L) that is the acline period. The example power supply 100 of FIG. 1 has an ac inputcurrent I_(AC) 104.

In the example power supply of FIG. 1, a full wave bridge rectifier 106produces a dc rectified voltage V_(RECT) 112 that is received by a dc-dcconverter 116. Rectified voltage V_(RECT) 112 is positive with respectto an input return 108. Dc-dc converter 116 has an input current I_(IN)114 that has a pulsating waveform with a period T_(S) that is theswitching period. The switching period T_(S) is much less than the acline period T_(L). The switching period T_(S) is the reciprocal of theswitching frequency, and the ac line period T_(L) is the reciprocal ofthe ac line frequency. In one example, the switching period T_(S) isabout 15 microseconds whereas the ac line period T_(L) is about 20milliseconds. In other words, the ac line period T_(L) is typicallyabout 1000 times greater than the switching period T_(S), so that thereare typically about 1000 switching periods within one ac line period.

In the example power supply of FIG. 1, a small capacitor C1 110 coupledacross the dc terminals of bridge rectifier 106 provides a low impedancesource for the pulses of input current I_(IN) 114. Capacitor C1 110filters the high frequency components of input current I_(IN) 114 suchthat the magnitude of the ac input current I_(AC) 104 at any instant issubstantially the average of the dc input current I_(IN) 114, theaverage taken over a switching period T_(S). Capacitor C1 110 is smallenough to allow the rectified voltage V_(RECT) 112 to becomesubstantially zero twice in every ac line period T_(L).

Dc-dc converter 116 in the example of FIG. 1 is controlled by acontroller 132 to regulate a substantially dc output current I_(O) 124that produces an output voltage V_(O) 126 at a load 128. Output voltageV_(O) 126 is positive with respect to an output return 130. In oneexample, load 128 is an arrangement of LEDs.

Dc-dc converter 116 typically includes at least one switch 118, at leastone coupled inductor 120, and at least one capacitor 122. All standardconverter configurations with pulsating input currents that aretypically used to provide galvanically isolated outputs, such as forexample the flyback converter and for example the many variants of thebuck converter may be realized by an arrangement of switches, coupledinductors, and capacitors represented by the dc-dc converter block 116in the example of FIG. 1.

The various components identified with the functions of the dc-dcconverter 116 and the controller 132 need not be confined to theboundaries suggested by the boxes drawn in the example power supply 100of FIG. 1. The individual components are segregated into easilyidentifiable regions in this disclosure to aid the explanation of theinvention. Therefore, for example, a component such as switch 118 maystill be considered an element of dc-dc converter 116 when switch 118 isphysically located with circuits associated with a different function.For example, switch 118 may be packaged together with bridge rectifier106, or switch 118 may be included with circuits of controller 132 in anintegrated circuit that is manufactured as either a hybrid or amonolithic integrated circuit.

In the example of FIG. 1, controller 132 receives input current sensesignal U_(IN) 134 that is representative of the dc input current I_(IN)114. Controller 132 also receives an input voltage sense signal U_(RECT)136 that is representative of the rectified input voltage V_(RECT) 112.Controller 132 also receives an output voltage sense signal U_(OSENSE)that is representative of the output voltage V_(O) 126.

Embodiments described in this disclosure may use many techniques tosense the input current I_(IN) 114 as the current sense signal U_(IN)134. For example, the input current may be sensed as a voltage on adiscrete resistor, or a current from a current transformer, or a voltageacross the on-resistance of a metal oxide semiconductor field effecttransistor (MOSFET) when the input current is the same as the current inthe transistor, or as a current from the sense output of a currentsensing field effect transistor (senseFET). Therefore, this disclosurewill omit specific examples of techniques to sense dc input currentI_(IN) 114.

In the example of FIG. 1, a switch 118 included in dc-dc converter 116is responsive to a drive signal 160 received from controller 132. In theexample of FIG. 1, drive signal 160 is a logic signal that may be highor low within a switching period T_(S). In one example, switch 118 isclosed when drive signal 160 is high, and switch 118 is open when drivesignal 160 is low. A closed switch is sometimes referred to as being inan on state. An open switch is sometimes referred to as being in an offstate. In other words, a switch that turns on closes, and a switch thatturns off opens. In the example of FIG. 1, the dc input current I_(IN)114 is a pulsating current that is substantially zero when drive signal160 is low.

It is appreciated that input current sense signal U_(IN) 134, inputvoltage sense signal U_(RECT) 136, and output voltage sense signalU_(OSENSE) 138 may be any signals that have a known relationship to thedc input current I_(IN) 114, the rectified input voltage V_(RECT) 112,and the output voltage V_(O) 126, respectively. For example, a voltagemay be sensed as a current signal, and a current may be sensed as avoltage signal.

Controller 132 includes an oscillator 144 that provides timing signalssuch as for example a clock signal 152 that sets the duration of theswitching period T_(S), and also may provide other timing signals notshown in FIG. 1. An arithmetic operator circuit 140 receives inputcurrent sense signal U_(IN) 134, input voltage sense signal U_(RECT)136, and output voltage sense signal U_(OSENSE) 138 to produce a scaledcurrent signal 146 that is the product of input current sense signalU_(IN) 134 multiplied by the ratio of the input voltage sense signalU_(RECT) 136 to the output voltage sense signal U_(OSENSE) 138, andmultiplied again by a constant scaling factor K₁.

Controller 132 also includes a resettable integrator 148. Resettableintegrator 148 integrates the scaled current signal 146 to produce theinput charge control signal U_(Q) 158. Thus, the arithmetic operatorcircuit 140 and resettable integrator 148 comprise an input chargecontrol signal generator. Input charge control signal U_(Q) 158 isdirectly proportional to the electrical charge received by dc-dcconverter 116 during a switching period. Input charge control signalU_(Q) 158 may be scaled by an additional constant scaling factor K₂. Inthe example of FIG. 1, resettable integrator 148 receives clock signal152 to reset the integrator and to initiate integration.

In one example, a resettable integrator 148 may include a capacitor, acurrent source, and a switch. The current source, with a valuerepresentative of the signal to be integrated, charges the capacitorduring the time of integration. The switch discharges the capacitor whenthe integrator is reset. Other examples of resettable integrator 148 mayinclude features of greater sophistication, including resetting theintegrator to a known value that is not necessarily zero, such that thecharging of the capacitor during the time of integration occurs in alinear operating range of the capacitor. In another example, resettableintegrator 148 may be a two-way integrator. That is, resettableintegrator 148 may integrate by charging a capacitor during oneswitching period T_(S) and may then integrate by discharging thecapacitor in a subsequent switching period. Such a two-way integratormay be useful in applications in which a high maximum duty ratio (e.g.,99%-100%) is desired for drive signal 160

In the example of FIG. 1, a delayed ramp generator 142 included incontroller 132 provides a delayed ramp signal U_(DR) 154. Delayed rampsignal U_(DR) 154 is typically a signal that includes piecewise linearsegments with characteristics chosen to achieve a desired power factorfrom a particular dc-dc converter. For a flyback converter, for example,the waveform of delayed ramp signal U_(DR) 154 may have a shorthorizontal segment of a magnitude greater than zero at the beginning ofthe switching period followed by a much longer segment that decreases tozero at a constant slope before the next switching period. For a buckconverter, for example, the waveform of delayed ramp signal U_(DR) 154may have two linearly decreasing segments at different slopes followingthe short horizontal segment. In one example, delayed ramp signal U_(DR)154 includes a first segment of substantially zero slope followed by asecond segment having a finite linear slope. The generation of thedelayed ramp signal is typically accomplished by summing portions oftriangular waveforms that are either generated for this purpose or arereadily available from other circuits in the controller.

A drive signal generator (i.e., comparator 156) in controller 132 of theexample of FIG. 1 compares input charge control signal U_(Q) 158 withdelayed ramp signal U_(DR) 154 to produce drive signal 160. In oneexample, drive signal 160 is at a high state when input charge controlsignal U_(Q) 158 is less than delayed ramp signal U_(DR) 154 and drivesignal 160 is at a low state when input charge control signal U_(Q) 158is greater than delayed ramp signal U_(DR) 154.

Functional blocks within controller 132 in the example of FIG. 1 may bearranged differently to operate on signals in a different order toproduce the same result, as shown in the example of FIG. 2. FIG. 2 showsan alternative arrangement of functions within controller 132. In theexample of FIG. 2, input current sense signal U_(IN) 134 is firstintegrated and scaled by resettable integrator 148 to produce an inputcharge signal U_(INQ) 205. Then, arithmetic operator circuit 140receives input charge signal U_(INQ) 205 to produce the input chargecontrol signal U_(Q) 158 that is compared to delayed ramp signal U_(DR)154 by comparator 156 as in the example of FIG. 1.

Individual signals within controller 132 in the examples of FIG. 1 andFIG. 2 may also be combined and arranged differently to produce the sameresult. For example, in an alternative arrangement to the directcomparison of delayed ramp signal U_(DR) 154 with input charge controlsignal U_(Q) 158, delayed ramp signal U_(DR) 154 may be subtracted frominput charge control signal U_(Q) 158, and the difference compared to aconstant level. Mathematically, this alternative just subtracts the samesignal (delayed ramp signal U_(DR) 154) from both inputs of comparator156.

FIG. 3 shows an example circuit 300 that may perform the functions ofthe arithmetic operator circuit of FIG. 1 and FIG. 2.

In the circuit of FIG. 3, bipolar NPN transistors 330, 320, 325, and 355are matched. To a very good approximation, the base to emitter voltageof a bipolar transistor is directly proportional to the naturallogarithm of the collector current. That is, for practical values in theregion of interest,

$\begin{matrix}{V_{BE} \approx {V_{T}{\ln\left( \frac{I_{C}}{I_{S}} \right)}}} & {{EQ}.\mspace{14mu} 1}\end{matrix}$where V_(BE) is the base to emitter voltage, V_(T) is the thermalvoltage fixed by physical constants, I_(C) is the collector current, andI_(S) is the reverse saturation current of the base to emitter junctionof the transistor. For the circuit in FIG. 3,V _(BE1) +V _(BE2) =V _(BE3) +V _(BE4)  EQ. 2Therefore, under the condition that the base current of all thetransistors is negligible, the relationship of Equation (1) requiresthat the currents I_(X) 305 and I_(Y) 360 are related by the expression

$\begin{matrix}{I_{Y} = {\frac{I_{C\; 2}}{I_{C\; 3}}I_{X}}} & {{EQ}.\mspace{14mu} 3}\end{matrix}$

In other words, application of the logarithmic relationship of Equation(1) to the circuit of FIG. 3 shows that the input current I_(X) 305 ismultiplied by the value I_(C2) of current sources 310 and 335. It canalso be shown that the input current I_(X) 305 is divided by the valueI_(C3) of the current sources 315 and 340. Therefore, multiplication oftwo signals may be achieved by the circuit in FIG. 3 when I_(X) 305 isproportional to a first signal while current sources 310 and 335 areproportional to a second signal. Multiplication by the reciprocal of athird signal may be achieved when the current sources 315 and 340 areproportional to the third signal. Many suitable variants of the examplecircuit of FIG. 3 are known in the art.

In one example, current sources I_(C2) 310 and 335 are variable currentsources controlled by the input voltage sense signal U_(RECT) 136, whilecurrent sources I_(C3) 315 and 340 are variable current sourcescontrolled by the output voltage sense signal U_(OSENSE). Thus, withinput current I_(X) directly proportional to the input charge sensesignal U_(INQ) 205, output current I_(Y) is representative of inputcharge control signal U_(Q) 158 according to the example of FIG. 2.

FIG. 4 is a timing diagram 400 of signals in the controller 132 of FIG.1 and FIG. 2 for two complete switching periods 405 and 410. Aconvenient reference signal for timing purposes is the clock signal 152.In the example timing diagram 400, the falling edge of clock signal 154marks the switching periods. For example, switching period 405 starts attime t₀ 415 and ends at time t₃ 430, whereas switching period 410 startsat time t₃ 430 and ends at time t₄ 435.

In the example timing diagram 400 of FIG. 4, drive signal (“GATE”) 160goes high at the beginning of switching period 405 to close a switch 118in dc-dc converter 116. Dc-dc converter 116 may receive dc input currentI_(IN) 114 while switch 118 is closed. Timing diagram 400 shows delayedramp signal U_(DR) 154 at a constant positive value for a delay timeT_(D) 440 after the start of switching period 405, then decreasinglinearly to zero at time t₂ 425 at the rising edge of clock signal 152.As shown in FIG. 4, delayed ramp signal U_(DR) 154 includes a firstsegment 445 of substantially zero slope followed by a second segment 450having a finite linear slope.

In the example of FIG. 4, the falling edge of clock signal 152 resetsresettable integrator 148 at times t₀ 415, t₃ 430, and t₄ 435, to bringinput charge control signal U_(Q) 158 to a value of zero. Dc-dcconverter 116 receives dc input current I_(IN) 114 when drive signal 160is high, as indicted by input current sense signal U_(IN) 134. Inputcharge control signal U_(Q) 158 increases as resettable integratorintegrates input current sense signal U_(IN) 134.

The example timing diagram 400 shows that drive signal 160 remains highwhen input charge control signal U_(Q) 158 is less than delayed rampsignal U_(DR) 154. Drive signal 160 goes low in FIG. 4 after inputcharge control signal U_(Q) 158 becomes equal to the value of delayedramp signal U_(DR) 154. In other words, switch 118 is closed from timet₀ 415 at the beginning of switching period T_(S) 405 until input chargecontrol signal U_(Q) 118 rises to reach the value of delayed ramp signalU_(DR) 154 at time t₁ 420. When input charge control signal U_(Q)reaches the value of delayed ramp signal U_(DR) 154 at time t₁ 420,switch 118 opens for the remainder of the switching period T_(S) 405.

Since delayed ramp signal U_(DR) 154 is decreasing at time t₁ 420, inputcharge signal U_(Q) 158 becomes greater than delayed ramp signal U_(DR)154 between time t₁ 420 and the end of switching period T_(S) 405.Therefore, controller 132 opens switch 118 when the value of inputcharge signal U_(Q) 158 becomes greater than the value of delayed rampsignal U_(DR) 154.

FIG. 5 shows one example of an integrated circuit controller 585 in apower supply 500 that includes a particular dc-dc converter known as aflyback converter. The example flyback converter of FIG. 5 includes anenergy transfer element that is a coupled inductor T₁ 535, sometimesreferred to as a transformer. Coupled inductor T₁ 535 has a primarywinding 525 that has one end coupled to the rectified input voltageV_(RECT) 112. Coupled inductor T₁ 535 has a secondary winding 530 thathas one end coupled to the output return 130. Coupled inductor T₁ 135has a sense winding 550 that has one end coupled to the input return108.

The example power supply 500 of FIG. 5 has switch S1 118 of the dc-dcconverter included in an integrated circuit controller 585. Switch S1118 in the example of FIG. 5 is coupled to one end of primary winding525. Switch S1 118 opens and closes in response to a drive signal 160.In one example, switch S1 118 may be a metal oxide semiconductor fieldeffect transistor (MOSFET). In another example, switch S1 118 may be abipolar junction transistor (BJT). In yet another example, switch S1 118may be an insulated gate bipolar transistor (IGBT). A clamp circuit 520is coupled across the primary winding 525 to limit the voltage acrossprimary winding 525 when switch S1 118 opens.

In the example power supply 500 of FIG. 5, controller 585 generates adrive signal 160 in response to an input voltage sense signal 136, anoutput voltage sense signal 138, and an input current sense signal 134.Any of the several ways practiced in the art to sense current in aswitch may provide the current sense signal 134. In the example powersupply of FIG. 5, input current sense signal 134 is a current I_(S) 565that is representative of the value of current ID 595 in switch S1 118.In the example power supply of FIG. 5, the current ID 595 is the same asthe dc input current I_(IN) 114 when switch S1 118 is closed.

Controller 585 in the example power supply 500 of FIG. 5 receives inputvoltage sense signal U_(RECT) 136 as a current I_(RECT) 590 that isrepresentative of the peak value of the rectified input voltage V_(RECT)112. Capacitor C2 510 charges through diode 505 to the peak value ofrectified voltage V_(RECT) 112. Capacitor C2 510 discharges throughresistor R1 515 at a rate that allows a negligible change in currentI_(RECT) 590 during half an ac line period T_(L). Therefore, the examplecontroller 585 in the example power supply 500 of FIG. 5 is responsiveto the peak of rectified input voltage V_(RECT) 112.

In the example power supply 500 of FIG. 5, the switching of switch S1118 produces a pulsating current in secondary winding 530. The currentin secondary winding 530 is rectified by diode D1 540 and filtered bycapacitor C3 545 to produce a substantially dc output voltage V_(O) 126and an output current I_(O) 135 provided to a load not shown in FIG. 5.

Coupled inductor T1 535 in the example power supply 500 of FIG. 5includes a bias winding 550. Current in bias winding 550 is rectified bydiode 555 and filtered by capacitor 570 to produce a substantially dcvoltage VB 570 that is representative of output voltage V_(O) 126.

Controller 585 in the example power supply 500 of FIG. 5 receives outputvoltage sense signal U_(OSENSE) 138 as a feedback current I_(FB) 575through feedback resistor R_(FB) 580 that is representative of outputvoltage V_(O) 126. With these inputs described for input current sensesignal U_(IN) 134, input voltage sense signal U_(RECT) 138, and outputvoltage sense signal U_(OSENSE) 138, controller 585 in the example powersupply 500 of FIG. 5 operates in the same way as the example controller132 of FIG. 1.

FIG. 6 is a flowchart 600 that describes a method to control a powersupply to generate a high power factor with a regulated output current.

After starting in step 605, input voltage and output voltage are sensedin step 615. Step 620 sets the initial value for an integration step.Next, a switch is closed in step 625 allowing input current to flow.While the switch is closed, the input current is sensed in step 630. Thesensed input current is scaled by the ratio of sensed input voltage tosensed output voltage in step 635. A delayed ramp signal is generated instep 640.

The scaled input current is integrated in step 645. The integral of thescaled input current is compared to the delayed ramp in step 650. If theintegral of the scaled input current is less than the delayed rampsignal, then the input current is allowed to continue to flow and theintegration continues in steps 625 through 650. If the integral of thescaled input current is not less than the delayed ramp signal, then theinput current is terminated in step 655, and the process continues tostep 615.

What is claimed is:
 1. A power supply, comprising: an energy transferelement coupled between an input of the power supply and an output ofthe power supply, wherein the output of the power supply is galvanicallyisolated from the input; a switch coupled to the energy transfer elementto control a transfer of energy from the input to the output of thepower supply; a controller coupled to the switch, wherein the controllerincludes: a delayed ramp generator configured to generate a delayed rampsignal; an integrator coupled to integrate an input current of the powersupply and to generate a first signal in response thereto; an arithmeticoperator coupled to generate second signal responsive to the firstsignal and responsive to a ratio of a rectified input voltage to a dcoutput voltage of the power supply; and a drive signal generator coupledto receive the delayed ramp signal and the second signal and to generatea drive signal in response thereto to control switching of the switch toprovide power factor correction of the power supply and to provide aregulated current at the output of the power supply.
 2. The power supplyof claim 1, wherein the delayed ramp signal includes a plurality ofpiecewise linear segments.
 3. The power supply of claim 2, wherein theplurality of piecewise linear segments includes a first segment ofsubstantially zero slope followed by a second segment having a finitelinear slope.
 4. The power supply of claim 1, wherein the controllerfurther comprises an oscillator coupled to generate a clock signal,wherein the delayed ramp signal is generated responsive to the clocksignal.
 5. The power supply of claim 1, further comprising an integratedcircuit, wherein the controller and the switch are included in theintegrated circuit.
 6. The power supply of claim 1, wherein the drivesignal generator includes a comparator coupled to turn on the switch inresponse to the second signal being less than the delayed ramp signaland to turn off the switch in response to the second signal beinggreater than the delayed ramp signal.
 7. A power supply, comprising: anenergy transfer element coupled between an input of the power supply andan output of the power supply, wherein the output of the power supply isgalvanically isolated from the input; a switch coupled to the energytransfer element to control a transfer of energy from the input to theoutput of the power supply; a controller coupled to the switch, whereinthe controller includes: a delayed ramp generator configured to generatea delayed ramp signal; an arithmetic operator coupled to generate afirst signal that is responsive to an input current of the power supplyand responsive to a ratio of a rectified input voltage to a dc outputvoltage of the power supply; an integrator coupled to integrate thefirst signal to generate a second signal in response thereto; and adrive signal generator coupled to receive the delayed ramp signal andthe second signal and to generate a drive signal in response thereto tocontrol switching of the switch to provide power factor correction ofthe power supply and to provide a regulated current at the output of thepower supply.
 8. The power supply of claim 7, wherein the delayed rampsignal includes a plurality of piecewise linear segments.
 9. The powersupply of claim 8, wherein the plurality of piecewise linear segmentsincludes a first segment of substantially zero slope followed by asecond segment having a finite linear slope.
 10. The power supply ofclaim 7, wherein the controller further comprises an oscillator coupledto generate a clock signal, wherein the delayed ramp signal is generatedresponsive to the clock signal.
 11. The power supply of claim 7, furthercomprising an integrated circuit, wherein the controller and the switchare included in the integrated circuit.
 12. The power supply of claim 7,wherein the drive signal generator includes a comparator coupled to turnon the switch in response to the second signal being less than thedelayed ramp signal and to turn off the switch in response to the secondsignal being greater than the delayed ramp signal.
 13. An apparatus,comprising: a load; and a power supply including an output that iscoupled to provide a regulated dc output current to the load, whereinthe power supply further includes: an energy transfer element coupledbetween an input of the power supply and the output of the power supply,wherein the output of the power supply is galvanically isolated from theinput; a switch coupled to the energy transfer element to control atransfer of energy from the input to the output of the power supply; acontroller coupled to the switch, wherein the controller includes: adelayed ramp generator configured to generate a delayed ramp signal; anintegrator coupled to integrate an input current of the power supply andto generate a first signal in response thereto; an arithmetic operatorcoupled to generate second signal responsive to the first signal andresponsive to a ratio of a rectified input voltage to a dc outputvoltage of the power supply; and a drive signal generator coupled toreceive the delayed ramp signal and the second signal and to generate adrive signal in response thereto to control switching of the switch toprovide power factor correction of the power supply and to provide theregulated dc output current at the output of the power supply.
 14. Theapparatus of claim 13, wherein the load includes a light emitting diode(LED).
 15. The apparatus of claim 13, wherein the power supply furthercomprises an integrated circuit, wherein the controller and the switchare included in the integrated circuit.
 16. The apparatus of claim 13,wherein the drive signal generator includes a comparator coupled to turnon the switch in response to the second signal being less than thedelayed ramp signal and to turn off the switch in response to the secondsignal being greater than the delayed ramp signal.
 17. An apparatus,comprising: a load; and a power supply including an output that iscoupled to provide a regulated dc output current to the load, whereinthe power supply further includes: an energy transfer element coupledbetween an input of the power supply and the output of the power supply,wherein the output of the power supply is galvanically isolated from theinput; a switch coupled to the energy transfer element to control atransfer of energy from the input to the output of the power supply; acontroller coupled to the switch, wherein the controller includes: adelayed ramp generator configured to generate a delayed ramp signal; anarithmetic operator coupled to generate a first signal that isresponsive to an input current of the power supply and responsive to aratio of a rectified input voltage to a dc output voltage of the powersupply; an integrator coupled to integrate the first signal to generatea second signal in response thereto; and a drive signal generatorcoupled to receive the delayed ramp signal and the second signal and togenerate a drive signal in response thereto to control switching of theswitch to provide power factor correction of the power supply and toprovide the regulated dc output current at the output of the powersupply.
 18. The apparatus of claim 13, wherein the load includes a lightemitting diode (LED).
 19. The apparatus of claim 13, wherein the powersupply further comprises an integrated circuit, wherein the controllerand the switch are included in the integrated circuit.
 20. The apparatusof claim 13, wherein the drive signal generator includes a comparatorcoupled to turn on the switch in response to the second signal beingless than the delayed ramp signal and to turn off the switch in responseto the second signal being greater than the delayed ramp signal.
 21. Acontroller for use in a power supply, the controller comprising: adelayed ramp generator coupled to generate a delayed ramp signal; aninput charge control signal generator to be coupled to receive: an inputcurrent sense signal that is representative of an input current of thepower supply, an input voltage sense signal that is representative of adc input voltage of the power supply, and an output voltage signal thatis representative of a dc output voltage of the power supply, whereinthe input charge control signal generator is configured to generate aninput charge control signal in response to an integral of the inputcurrent sense signal and in response to a ratio of the input voltagesense signal to the output voltage sense signal; and a drive signalgenerator configured to generate a drive signal to control a switch ofthe power supply to regulate an output of the power supply in responseto the input charge control signal and the delayed ramp signal.
 22. Thecontroller of claim 21, wherein the delayed ramp signal includes aplurality of piecewise linear segments.
 23. The controller of claim 22,wherein the plurality of piecewise linear segments includes a firstsegment of substantially zero slope followed by a second segment havinga finite linear slope.
 24. The controller of claim 21, wherein the drivesignal generator generates the drive signal to control the switch of thepower supply to regulate an output current of the power supply.
 25. Thecontroller of claim 21, wherein the input charge control signalgenerator further comprises: an integrator coupled to integrate theinput current sense signal to generate an input charge signal; and anarithmetic operator circuit coupled to multiply, divide or scale theinput voltage sense signal, the output voltage sense signal and theinput charge signal to generate the input charge control signal.
 26. Thecontroller of claim 21, wherein the input charge control signalgenerator further comprises: an arithmetic operator circuit coupled togenerate a scaled current signal that is a product of the input currentsense signal multiplied by the ratio of the input voltage sense signalto the output voltage sense signal, the product further multiplied by ascaling factor; and an integrator coupled to integrate the scaledcurrent signal to generate the input charge control signal.